The present invention relates to drying wet material within a drying chamber. In particular, the present invention relates to drying wet material within an oscillating drying chamber under vacuum such that a rotary vacuum seal is not required.
In the chemical process industries, it is frequently necessary to dry granulated or powdered materials that are wet with one or more solvents. The materials to be dried may be the product of precipitation of solids from solution, such as in fractional crystallization, or may have become wet as the result of one or more washing procedures. The materials to be dried may also have become wet as the result of a coating process in which a solution or suspension of one or more substances is mixed with or sprayed onto the material to be coated, thereby necessitating a drying step before further processing. Sometimes granulated or powdered materials absorb moisture or solvent vapors from an atmosphere containing these materials. The absorbed moisture or solvent must be removed in order to render the powdered or granulated material suitable for its intended use.
A variety of drying chambers, ovens, etc. have been developed over the years in answer to the needs of the chemical and process industries. The drying method employed by these dryers generally involves passing a flow of relatively dry air past or through material while exposing fresh surface area. Wet material is exposed by stirring or tumbling the material in a rotating drier, or by passing air through the material in a fluidized bed.
For very hygroscopic materials and materials wet with a high boiling point (low vapor pressure) solvent, simple drying in a current of dry air may not be sufficient to remove the moisture from the powdered or granulated material. Where simple air drying is insufficient, the temperature of the air, material, or both may be raised in order to increase the vapor pressure of the solvent wetting the powdered or granulated material. Increase of vapor pressure accelerates evaporation of the solvent.
There have been many types of dryers or evaporators developed for the purpose of drying powdered or granulated materials that supply heat input during the drying process. Some evaporators (dryers) make use of a simple heated chamber, with or without stirring of the material to be dried. Some evaporators make use of a heated interior wall combined with rotation to help drive off the solvent by exposing fresh surface area, etc.
The drying of powders or granulated materials may be complicated if the material to be dried is highly hygroscopic or wet with a high boiling point (low vapor pressure) solvent and is also sensitive to thermal degradation. For example, some pharmaceutical agents are heat-sensitive and cannot be dried by simple heating without severe degradation. Another example of a material that cannot easily be heated to drive off moisture is metal or ceramic powder that has been coated with a solution of a binder or lubricant. Binders aid in pressing powdered material into pellets and, after further consolidation via a thermal sintering step, are useful as filter elements, electrolytic capacitor anodes, etc. Simple heating, even with tumbling, may melt the solid binder/lubricant as the moisture or solvent evaporates. This results in the production of large agglomerates, or clumps, which are unsuitable for downstream processing.
In order to address the problem of drying heat-sensitive materials, vacuum is enlisted to facilitate rapid evaporation of the solvent at a lower temperature than is possible at standard atmospheric pressure. The use of vacuum without agitation is sufficient to thoroughly dry powders or granulated solids in thin layers, but experience has demonstrated that thick layers of material dry very slowly under these conditions. This appears to result from the increase in pressure on the material below the surface due the weight of the material above. Thus, agitation of the material to be dried is employed in order to expose fresh surface area to vacuum. While agitation may be accomplished by the use of impeller blades to stir the material, in practice agitation is usually accomplished by use of a rotary evaporator. A rotary evaporator features a rotating vacuum chamber containing the material to be dried, and connects to a vacuum source through a rotary vacuum seal. FIG. 1 displays a typical rotary vacuum evaporator of the prior art. The vacuum evaporator contains a rotary chamber 101. The top of the chamber has an outlet neck 102. A fixed vacuum line 103 is placed into the neck and attached with a rotating chamber clamp 104. A rotating vacuum seal 5 and a fixed vacuum seal 106 are used to create an air-tight seal. The chamber is connected to a rotating drive housing 107 and a rotating drive gear 108 to rotate chamber 101 almost in the horizontal plane. Above the gear and attached to the fixed vacuum line 103 through fixed chamber clamp 110 is a fixed housing 109. Arrows 111 depict the direction of the vapor path to the vacuum pump. The xe2x80x9cwet materialxe2x80x9d is poured into 101 before connection to the rotary seal/rotary drive mechanism.
Heat may be applied by immersing the lower portion of 101 in a heated fluid such as hot water or oil. Heat is generally supplied to the material contained within the rotary evaporator to compensate for cooling due to the latent heat of vaporization of the solvent absorbed as it evaporates. The amount of heat required and the rate of input of thermal energy necessary to compensate for evaporative cooling may be calculated very accurately based upon the latent heat of vaporization of the solvent and the desired drying time for the amount of solvent to be removed.
Although rotary vacuum evaporators have been used successfully for many years in drying heat-sensitive powders or granulated materials, there are problems associated with the use of rotary vacuum seals. Rotary vacuum seals are necessary to communicate vacuum from the vacuum pump to the rotary chamber containing the material to be dried. During the drying process, evaporation of the solvent tends to be rapid (generally, process cost is reduced as the rate of evaporation is increased). Rapid evaporation of the solvent tends to displace some of the material being dried from the wet bulk material to an empty portion of the vacuum chamber over the bulk material. The effect is sufficiently pronounced that, with glass or other transparent vacuum vessels, the completion of the drying step may be readily detected as cessation of the agitation of the powder surface due to evaporation of the solvent as the vacuum vessel rotates.
Evaporation of the solvent from wet material in a rotary evaporator also results in transport of a portion of the material being dried towards the source of vacuum via vapors of the evaporating solvent. For applications involving drying of abrasive materials, such as tungsten carbide powder (used to fabricate cutting tools), tantalum powder (used to fabricate electrolytic capacitor anodes), etc., vapor transport of these materials to the seal area results in rapid wear of the rotary vacuum seal. Wear of the vacuum seal causes loss of vacuum integrity and process efficiency, as well as contamination of the material being dried by particles of the degraded seal. Filters may be used to extend the life of the rotary vacuum seal, but the fundamental problem of seal wear remains. Further, rapid wear of the vacuum seal necessitates frequent replacement of the seal, which increases process costs and machine down time. Accordingly, what is needed is a drying method and apparatus that permits vacuum enhanced drying, yet is free of the shortcomings associated with use of a vacuum seal.
It is often desirable to dry powdered or granulated materials that are wet with one or more solvents. These materials may be particularly hygroscopic materials and/or materials wet with a high boiling point (low vapor pressure) solvent and which are sensitive to heat. It is further desirable to dry such materials in an oscillating chamber, which encloses the wet material, wherein vacuum is applied to the chamber to expedite solvent evaporation at a pressure reduced from that of standard atmospheric pressure.
The invention is directed to an oscillating vacuum chamber dryer for drying wet material that does not require use of a rotary vacuum seal. The invention is further directed to a method for drying wet material in an oscillating vacuum chamber dryer that does not require use of a rotary vacuum seal.
Specifically, the present invention is directed to an oscillating vacuum chamber dryer for drying wet powdered or granulated materials comprising: a drying chamber having a vacuum port, the vacuum port coupled to a vacuum source to produce a vacuum in the drying chamber, the drying chamber coupled to an oscillation mechanism configured to cause the chamber to reversibly rock back and forth in an oscillatory manner.
The invention is further directed to a process for drying a coated powdered or granulated material comprising introducing the material into a chamber of an oscillating vacuum chamber dryer, supplying a liquid to the chamber, rocking the chamber back and forth in an oscillatory manner to cause the material and liquid to flow back and forth from one end of the chamber to the other, allowing the liquid to coat the material and continuously exposing fresh surface area, and, while maintaining the rocking, applying a vacuum to the chamber through a vacuum port until the material has dried.